1. Field of the Invention
This invention relates to a process for improving the low-temperature stability of aqueous anionic surfactant solutions by addition of a mixture of selected nonionic surfactants, to manual dishwashing detergents containing these mixtures and to the use of this mixture for the production of aqueous anionic surfactant solutions having improved low-temperature stability.
2. Statement of the Related Art
Most manual dishwashing detergents contain anionic surfactants as their active components. Typical primary surfactants are alkylbenzene sulfonates, secondary alkane sulfonates, fatty alcohol ether sulfates and alkyl sulfates. These surfactants are present in the formulations in total concentrations of up to about 30% by weight, high-performance synergistic combinations essentially being used. Suitable co-surfactants or secondary surfactants are, for example, betaines, fatty acid alkanolamides, amine oxides and ether carboxylic acids which are used in much smaller quantities. Their function is to increase washing power and foam stability (cf. Seifen-Ole-Fette-Wachse 115, 149 (1989)).
One of the problems involved in the formulation of such detergents lies in the comparatively poor low-temperature stability of aqueous anionic surfactant solutions. Accordingly, unwanted clouding can occur, particularly when formulations or part-formulations have to be stored for a certain time before use. If it is stored outside, the product can even solidify.
In this connection, foaming detergent mixtures of anionic surfactants, alkyl oligoglucosides and, optionally, amine oxides and their use as dishwashing detergents are proposed in European patents EP-B 0 070 074, EP-B 0 070 075, EP-B 0 070 076 and in EP-B 0 075 995 and EP-B 0 075 996 (Procter & Gamble). However, the low-temperature stability of the mixtures is not significantly improved by the addition of the nonionic surfactants mentioned.
In addition, German patent application DE-A1 40 25 065 (Henkel) describes aqueous surfactant mixtures which, in addition to alkyl oligoglucosides and mixtures of long-chain and short-chain alkyl sulfates, may also contain fatty alcohol polyethylene glycol ethers, preferably adducts of 3 to 10 mol ethylene oxide with C.sub.10-20 fatty alcohols. The surfactant compounds are used as premixes in the production of liquid detergents. However, this patent application does not refer to the low-temperature stability of the mixtures or to their advantageous use in manual dishwashing detergents.
Accordingly, the problem addressed by the present invention was to provide a process for the production of aqueous anionic surfactant solutions which would be free from the disadvantages mentioned above.